Inhibition of nitrogenase activity and nodule oxygen permeability by water deficit

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Abstract
Short-term effects of water deficit on nitrogenase activity were investigated with hydroponically grown soybean plants (Glycine max L. Merr. cv. Biloxi) by adding polyethylene glycol (PEG) to the hydroponic solution and measuring nitrogenase activity, nodule respiration, and permeability to oxygen diffusion (Po). These experiments showed a rapid decrease in acetylene reduction activity (ARA) and nodule respiration. A consequence of the decreased respiration rate was that Po calculated by Fick's Law also decreased. However, these results following PEG treatment were in direct conflict with a previous report of stability in Po determined by using an alternative technique. To resolve this conflict, an hypothesis describing a sequence of responses to the initial PEG treatment is presented. An important finding of this study was that the response to water deficit induced by PEG occurred in two stages. The first stage of decreased nodule activity was O2-limited and could be reversed by exposing the nodules to elevated pO2. The second stage which developed after 24 h of exposure to PEG resulted in substantial loss in nodule activity and this activity could not be recovered with increased pO2. Severe water deficit treatments disrupt nodule activity to such a degree that O2 is no longer the major limitation.

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